Rhyacophila is one of the largest genera of caddisflies, containing about 465 species. Approximately 100 species are found in North America with 83 occurring in the west, The larvae of 24 western North American species are associated for the first time, making a total of 38 species for which larvae are now associated. Two more larvae are tentatively associated with the adults. Larvae of another 19 species, although at present unassociated, are so closely related to other species in the adult stage that it is probable that the larvae are nearly identical to various of the described larvae. The immature stages were associated with the adult by use of the metamorphotype method. The larval descriptions are then based on the sclerites of the metamorphotype and on the corresponding mature larvae from the same area. Laboratory rearing of eggs, larvae, and pupae was used on a small scale. The known Rhyacophila larvae in western North America have been described and figured. A key was developed for the mature larvae of the associated species of the subfamily Rhyacophilinae occurring in western North America. Eggs of a Nearctic species of Rhyacophila (R. norcuta Ross) are described for the first time, The published data and the newly acquired information on the distribution and bionomics of Rhyacophila spp. in western North America is discussed. Characteristics of the larvae were found to support the classification system of Schmid (1970) better than that of Ross (1956). In most instances larvae can be readily placed in Schmid's species groups, However, in terms of his hierarchy of divisions and branches, the larvae often cannot be grouped in the adult classification system.